Multiple antibiotic resistance in clinical isolates of Streptococcus pneumoniae will be examined in terms of the nature, organization, and probable origin of the genes that confer the resistances, and with respect to the mechanisms by which they are spreading by transfer within and between species. Many of the resistances are due to R-determinants that are present in large insertions into the chromosomes. None have been found on plasmids. Some of the insertions transfer by conjugation to the chromosomes of other cells, and this is the first example of conjugative transfer of chromosomal elements in gram positive bacteria. Interspecies conjugation has been seen from other streptococcal groups, and this will be studied further. Molecular mechanisms of the transfer process will be examined in terms of intermediate structures, whether the chromosome is mobilized, whether transposition occurs, and the nature of the transfer genes compared to those in other systems. A major part of this study will be approached by restriction mapping, using cloning of parts of the regions and a variety of plasmid probes and transformation assays to locate and characterize the genes.